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The Internet is revolutionizing business by providing an affordable and efficient way to link companies with their partners as well as customers. Nevertheless, there are problems that degrade the profitability of the Internet: closed markets that cannot use each other's services; incompatible applications and frameworks that cannot interoperate or built upon each other; difficulties in exchanging business data. Web Services is a new paradigm for e-business that is expected to change the way business applications are developed and interoperate. A Web Service is a self-describing, self-contained, modular application accessible over the web. It exposes an XML interface, it is registered and can be located through a Web Service registry. Finally, it communicates with other services using XML messages over standard Web protocols. This paper presents the Web Service model and gives an overview of existing standards. It then sketches the Web Service life-cycle, discusses related technical challenges and how they are addressed by current standards, commercial products and research efforts. Finally it gives some concluding remarks regarding the state of the art of Web Services.